1. Field of the Invention
This invention relates to a process for preparing oxabicycloalkane compounds and to the compounds whenever prepared by the process and their use as intermediates in the preparation of herbicidally active oxabicycloalkane compounds
2. Description of the Prior art
European Patent Application No. EP-A-81893 discloses a broad class of oxabicycloalkane compounds having herbicidal activity, together with various classes of intermediates and processes whereby the herbicidally active compounds may be prepared.
One subclass of the oxabicycloalkane compounds disclosed in EP-A-81893 comprises oxabicyclo[2.2.1]heptane compounds which may be represented by the general formula ##STR1## wherein R.sup.1 is a hydrogen atom or C.sub.1-6 alkyl group optionally substituted by up to 3 fluorine, chlorine and/or bromine atoms; R.sup.2 is a hydrogen atom or a C.sub.1-6 straight-chain alkyl group; R.sup.3 is a hydrogen atom; a C.sub.1-10 alkyl group; a cyano group; an alkyl group substituted by one or more halogen atoms or by a hydroxy group, a cyano group, a C.sub.1-6 alkoxy group, an aryloxy group, a C.sub.1-6 alkylsulphonyl group, an arylsulphonyl group, an aralkylsulphonyl group, an azido group, a C.sub.1-6 alkoxycarbonyl group, an aralkoxy carbonyl group, a hydroxycarbonyl group, a phosphoryl group, a phosphoryloxy group, or an amine oxide, carbamoyl or thiocarbamoyl group in which each nitrogen is substituted by hydrogen or by 1 or 2 C.sub.1-4 alkyl groups; a C.sub.2-4 alkenyl or alkynyl group; an aryl or aralkyl group, each containing from 6 to 11 carbon atoms including 1 to 4 carbon atoms in the alkyl portion and optionally ring substituted by one or more fluorine, chlorine and/or bromine atoms or by a C.sub.1-2 alkyl or alkoxy group, each optionally substituted by one or more fluorine and/or chlorine atoms; a group --CSNH.sub.2 ; a group --CO.sub.2 R.sup.6 or --CON(R.sup.6).sub.2 in which R.sup.6 is a hydrogen atom or a C.sub.1-6 alkyl group; or a C.sub.1-6 acyl group or an oxime or an acetal derivative of said acyl group; each R.sup.4 is independently a hydrogen atom; an alkyl group optionally substituted by up to 3 halogen atoms; a hydroxy group; or a C.sub.1-4 alkoxy group; each R.sup.5 independently is a hydrogen atom, a C.sub.1-4 alkyl group optionally substituted by up to 3 halogen atoms, or a chlorine or bromine atom; both of Q are hydrogen atoms or flurine atoms; and W is an optionally-substituted unsaturated group of up to 4 carbon atoms; an optionally substituted aryl or heterocyclic group containing up to 14 carbon atoms; a C.sub.3-10 cycloaliphatic group optionally substituted by C.sub.1-3 alkyl; or a C.sub.3-10 secondary alkyl group.
EP-A-81893 describes the preparation of the compounds of formula III by a process which comprises reacting an oxabicycloalkane compound (a 7-oxabicyclo[2.2.1]heptan-2-ol) of formula ##STR2## wherein R.sup.1, R.sup.2, R.sup.3, R.sup.4 and R.sup.5 are as defined above, with a compound of formula WCQ.sub.2 L wherein W and Q are as defined above and L represents a leaving group, for example a halogen atom, especially a bromine, chlorine or iodine atom, or an organic sulphonyloxy group, for example a mesyloxy or tosyloxy group.
The reaction is preferably carried out in the presence of a strong base, for example an alkali metal hydride, hydroxide or carbonate, for example sodium hydride, sodium hydroxide or potassium carbonate. Preferably an inert solvent is used; typical solvents include ethers, sulphoxides, aromatic hydrocarbons and chlorinated hydrocarbons, for example diethyl ether, tetrahydrofuran, dimethyl sulphoxide, toluene and methylene chloride. It may be desired to include a catalyst for the reaction; suitable catalysts include organic bases, such as tertiary amines and quaternary ammonium compounds, for example triethylamine or tetrabutylammonium iodide. Suitable temperatures for the reaction are, for example from 0.degree. to 120.degree. C., preferably from 20.degree. to 100.degree. C.
The compounds of formula I are prepared by epoxidation-cyclisation of the corresponding unsaturated cyclic alcohol, i.e. a cyclohex-3-en-1-ol compound of formula ##STR3## wherein R.sup.1, R.sup.2, R.sup.3, R.sup.4 and R.sup.5 are as defined above.
The epoxidation is effected by action of a peroxide, such as m-chloroperbenzoic acid, peracetic acid, tert-butyl hydroperoxide (TBHP) or equivalent peroxide reagents. Preferably, an oxidation with TBHP is conducted in the presence of an appropriate transition metal catalyst. Suitable transition metal catalysts are complexes of metals of atomic numbers 22-31, 40-49 and 72-81. Preferably, the complex is an organic complex, for example with beta-diketones, o-hydroxybenzaldehydes or o-hydroxybenzophenones and particularly with acetylacetone. While any of these transition metal catalysts can be used, those of vanadium or molybdenum are preferred; for example, vanadium (IV) bis(2,4-pentanedionate)oxide is preferred. The reaction is suitably conducted in the presence of an inert solvent such as a chlorinated hydrocarbon, ether or hydrocarbon.
The resulting product epoxy-alcohol may be purified or may be cyclised without isolation.
The cyclisation step is carried out, particularly when the peroxide used to effect epoxidation is non-acidic, by treating the epoxy-alcohol with an acid, preferably a relatively strong acid such as sulphuric or a sulpnonic acid, such as methanesulphonic acid, benzenesulphonic acid or, especially, p-toluenesulphonic acid. The reaction is suitably carried out in a solvent of the type previously described for use in the preparation of the epoxy-alcohol.
The compounds of formula II are either known materials from natural sources or may be synthesised as described in EP-A-81893.
The present invention provides a readily effected single-step process for the preparation of oxabicycloalkane compounds of formula I which avoids the necessity for peroxide reagents.